In recent years, portable tools such as pocket telephones, compact video cameras and notebook-type personal computers have developed remarkably, and demands for a compact secondary battery such as Ni-hydrogen secondary battery or lithium secondary battery, as a powder source therefor are increasing.
In particular, a nonaqueous solvent-type lithium secondary battery using lithium, which is the basest metal, may allow realization of a compact lightweight high energy density battery. Accordingly, research and development thereof are proceeding aggressively.
A lithium secondary battery using metal lithium as a negative electrode is, however, disadvantageous in that dendritic acicular lithium crystals are readily generated and break through the separator and cause short-circuits.
As an effective means for solving this problem, a lithium ion secondary battery using a carbonized or graphitized carbon material for the negative electrode and a nonaqueous solvent containing lithium ion for the electrolytic solution has been proposed and is used in practice.
More specifically, the charge and discharge reaction is designed to take place in such a manner that when the carbon is doped, intercalated or the like by lithium ion, charging occurs, whereas when the lithium ion is dedoped or deintercalated, discharging occurs, so that the metal lithium can be prevented from precipitating and can be completely used. This reaction mechanism is being aggressively studied but has not yet been completely elucidated.
In the lithium secondary battery, carbon materials such as natural graphite, artificial graphite, pitch type carbon particles, pitch type carbon fibers, vapor grown carbon fibers, or non-graphitizable products baked under low temperature, are used as the negative electrode, and lithium is used as an active material thereof.
In order to increase the discharge capacity of lithium batteries, the amount of lithium taken in into the carbon must be increased as much as possible. On the other hand, the lithium taken in must be easily released. These intake and release actions preferably proceed smoothly and are not subject to any large change in the balance therebetween even if the actions are repeated. When this is successfully accomplished, a high current efficiency and a long cycle life can be attained.
It is said that as the graphite has a higher crystallinity, the amount of lithium taken in into a carbon (graphite) material increases. In general, the graphite can have further improved crystallinity when the graphitization temperature is higher. However, the graphitization temperature is generally about 3,200° C. at the highest and there is a definite limit to improving the crystallization of graphite by only temperature regulation. In order to solve this problem, JP-A-8-31422 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a technique of adding boron (B) to carbon powder and graphitizing the mixed powder, thereby elevating the crystallinity. Also, JP-A-9-259886 discloses a technique of processing a specific carbon to thereby improve the crystallinity of the graphite powder and reduce the specific surface area.
In a lithium battery, graphite powder formed into a paste using a binder is coated on a metal foil, a metal mesh or the like to form an electrode (negative electrode). The powder used preferably has a small specific surface area. The reasons therefor are described below. A passive film comprising an electrolytic solution or the like is formed on the surface of the graphite powder and as the specific surface area of the powder becomes larger, the production of the passive film increases. The passive film is a significant cause of reduction of the use efficiency of lithium. When the carbon material has a high activity, the electrolytic solution is decomposed, resulting in a short cycle life. Therefore, the activity of the carbon material is preferably decreased as much as possible. For this purpose, a carbon material having a lower chemical activity (reactivity) and a smaller specific surface area is preferably employed. Also, when the specific surface area of the powder becomes large, the amount of binder used in the formation of the powder into a poled plate increases and the coverage of binder on the graphite particles proportionally increases. As a result, the contact ratio between the graphite particles and the electrolytic solution is reduced and the charge and discharge capacity decreases.
As the capacity of the battery increases, the battery charges and discharges more current, and the required conductivity of the electrode increases compared with those of conventional batteries. More specifically, when the material requires a large amount of binder because of the low conductivity or poor coating performance, the resistance of the electrode plate itself is increased. As a result, this causes not only a decreases of the discharge capacity and Coulomb efficiency, but also a increase of heat generation and partial heat generation, as well as the possibility of dendrite generation, which is not preferable from the view point of safety. Accordingly, it is necessary to develop a carbon material which has the high conductivity of carbon itself, excellent coating performance, and increased charge and discharge capacity.
The method for obtaining graphite powder includes a method of pulverizing coke and the like and then graphitizing the powder and a method of graphitizing coke or the like and then pulverizing it. JP-A-6-295725 employs the latter method. When graphite is first formed and then pulverized, the graphite becomes highly crystallized having increased hardness and strength, which makes pulverization thereof difficult. In addition, the pulverization of graphite requires large force. When a large force is applied in pulverization, a greater amount of fine powder is produced, the particles are more liable to have scaly shapes, and the aspect ratio is increased. Due to the increased aspect ratio, more particles of flat shaped crystals are included, the specific area of the powder is increased, and the battery performance decreases.
The discharge capacity may be increased by increasing the crystallinity of the graphite and reducing the specific surface area of the graphite powder. However, not only these factors but also the permeability of the electrolytic solution, attributable to the shape of graphite powder, or the filling ratio of graphite particles during formation into an electrode, affect the battery properties. In the patent publications described above, these problems are not specifically addressed. In addition, the activity (reactivity) of the graphite powder, conductivity or the like should be considered.